1. Field of the Invention
The present invention relates to a planar packaged optical module, and in particular a planar packaged optical module which reciprocally or unilaterally converts electrical signals and optical signals.
2. Description of the Related Art
A planar packaged optical module is an electronic part which reciprocally or unilaterally converts electrical signals and optical signals, and is principally used in key equipment for optical communications. Such a planar packaged optical module generally has a structure like the one illustrated in FIGS. 4 and 5A.
That is, a planar packaged optical module generally has a housing 50 formed of epoxy resin or the like with a lid (in FIG. 5A, the lid is indicated with a broken line 62), an Si substrate 52 provided within the housing 50, a semiconductor laser device 54 (hereinafter referred to as an LD) provided on the Si substrate 52, a ferrule 56 which is adhered by an adhesive to the housing in a state in which the ferrule 56 is passed through the housing 50, and an optical fiber 58 which is adhesively retained within the ferrule 56 and is adhered to the Si substrate 52 in a state in which the optical fiber 58 is disposed so that one end surface of the optical fiber 58 opposes an active layer of the LD 54.
The Si substrate 52 is provided with an optical fiber mounting portion 52a and an LD mounting portion 52b which are separated by a separation groove 52c. A V-groove for mounting the optical fiber is formed on the optical fiber mounting portion 52a along a direction perpendicular to the separation groove 52c. The LD 54 which converts electrical signals inputted from the outside into optical signals is fixedly provided on the LD mounting portion 52b. 
The optical fiber 58 is disposed at a position such that one end surface of the optical fiber 58 precisely opposes the active layer of the LD 54. The optical fiber 58 is retained within the ferrule 56 at a notch portion of the housing 50. Within the housing 50, in a state in which a glass plate 60 which prevents the optical fiber 58 from rising up out of the V-groove is provided, the contact surface of the optical fiber 58 is adhesively fixed in the V-groove (not illustrated), which is provided on the optical fiber mounting portion 52a, by an adhesive 67 of UV hardenability or the like. The end surface of the optical fiber 58 which opposes the active layer of the LD 54 is stress-ruptured to a mirror-finished surface, while the other end surface of the optical fiber 58 is polished to a convex spherical surface.
The ferrule 56 which retains the optical fiber 58 at a notch portion of the housing 50 is structured to be substantially cylindrical. The ferrule 56 adhesively retains at the inner peripheral surface thereof the optical fiber 58 with an adhesive, while the outer peripheral surface of the ferrule 56 is fixed to the housing 50 with an adhesive. The adhesive which fixes the ferrule 56 to the housing 50 and the adhesive which fixes the optical fiber 58 to the inside of the ferrule 56 are epoxy resin adhesives which have a glass-transition temperature Tg of about 125xc2x0 C.
In the planar packaged optical module of the structure described above, lead portions 72 are placed on electrodes 82 of a distribution substrate 80 with solder balls 84 interposed between the lead portions 72 and the electrodes 82, as illustrated in FIG. 6A. Thereafter, the electrodes 82 of the distribution substrate 80 and the lead portions 72 are electrically bonded by leaving them for about thirty seconds in a high-temperature environment at 240xc2x0 C., as illustrated in FIG. 6B.
In the planar packaged optical module of the structure described above, the housing and the ferrule, and the ferrule and the optical fiber are respectively adhered to one another with an adhesive made of a resin, and the glass-transition temperature Tg of these adhesives is of a range of from about 100xc2x0 C. to about 125xc2x0 C.
When the electrodes 82 of the distribution substrate 80 are bonded with the lead portions 72 by solder balls or the like in a high-temperature environment at 240xc2x0 C. for a bonding time of thirty seconds, the adhesive which binds the housing and the ferrule and the adhesive which binds the ferrule and the optical fiber undergo glass-transition and harden once they have softened at almost the same time.
Because the adhesive which binds the housing and the ferrule and the adhesive which binds the ferrule and the optical fiber are fixed once they have softened at virtually the same time, sometimes the optical fiber slips out of position in the direction in which it is drawn into the housing at the time the adhesives have softened, as illustrated in FIG. 5B.
When such positional displacement occurs, the output variance from the optical fiber ends up exceeding 1 dB and noise increases. Therefore, there has been the drawback in that bonding by a reflow soldering which uses solder balls or the like is difficult.
In order to overcome such drawbacks, an object of the present invention is to provide a planar packaged optical module in which shifting of an optical fiber and a ferrule from their design positions and positional displacement of the optical fiber within the ferrule resulting from a movement of the optical fiber therein can be prevented, and which is suitable for reflow soldering.
In order to achieve this object, the invention according to the first aspect is a planar packaged optical module which includes a conversion device provided on a substrate within a housing and when supplied with one of an electric signal and an optical signal substantially converts one into the other; an optical fiber retained by a retaining member adhered to the housing and fixed on the substrate such that an end surface of the optical fiber opposes the conversion device; a first adhesive which adheres the housing and the retaining member to one another, the first adhesive substantially formed of a resin having a predetermined glass-transition temperature; and a second adhesive which adheres the retaining member and the optical fiber to one another, the second adhesive substantially formed of a resin having a glass-transition temperature greater than that of the first adhesive.
In the invention according to the first aspect, the glass-transition temperature of the second adhesive, which adheres the retaining member and the optical fiber to one another, is higher than the glass-transition temperature of the first adhesive, which adheres the housing and the retaining member to one another. Therefore, the time at which the adhesive between the housing and the retaining member softens and hardens and the time at which the adhesive between the retaining member and the optical fiber softens and hardens are staggered. In particular, the second adhesive which adheres the retaining member and the optical fiber to one another softens and hardens after the first adhesive which adheres the housing and the retaining member to one another softens and hardens.
Accordingly, displacement of the optical fiber due to movements of the optical fiber within the retaining member in an environment having a temperature higher than the glass-transition temperature of the adhesives at the time of reflow soldering and the like can be prevented, and the planar packaged optical module becomes suitable for reflow soldering.
It is preferable that the adhesives are thermoplastic resin adhesives. It is further preferable that the thermoplastic resin adhesives are epoxy resin adhesives.
As described above, according to the first aspect of the invention, effects can be obtained in that shifting of the retaining member from its design position in a high-temperature environment at the time of reflow soldering, and positional displacement of the optical fiber caused by a movement of the retaining member, can be prevented.
For this reason, because the planar packaged receptacle optical module can accommodate reflow soldering, an effect can be obtained in that, after reflow soldering, a state can be maintained in which the optical fiber and the active layer of the LD are aligned and opposed to one another with high precision.